Liquid Desander – Hydraulic Turndown Performance (Example) (B-FSM063)

The best way to demonstrate the information shown in the last two posts is via an example. The graphic below shows the performance of a liner style desander operating at liquid-only conditions for a period of 45 minutes.

The operating conditions are;

Pressure and temperature at inlet of desander: 100 psig and 60°C
Desander liner: 3” (76 mm) diameter
Number of liners: 10 (packed into a 30” diameter, 150# rated vessel)
Process fluid: produced water at 1060 kg/m³ density and 0.67 cP viscosity
Solid particles in fluid at 100 ppmv concentration

Design Condition

The desander is designed to treat 25,000 BWPD at a 25 psi pressure drop. At this design condition, the desander has a separation size of 20 microns.

Process Upset (Low)

Changes in the upstream FWKO cause the produced water rate to drop down to 11,000 BWPD at 19 minutes. At this flow the desander now operates at 5 psi pressure drop. Due to the lower pressure drop, the separation size coarsens to 32 microns. This low flow rate lasts until 25 minutes, then returns to the design condition.

Process Upset (High)

At 30 minutes the FWKO pushes out a slug of water, increasing the flow to the desander to 35,000 BWPD. The pressure drop increases to 50 psi and the separation size reduces 17 microns. This condition lasts until 40 minutes, whereupon the flow drops back to the design case.

So flow rate increase or decrease changes the pressure drop in a directly proportional manner, while changing the separation size in the opposite direction. As slugs of water (i.e. temporary increase in flow) pass through the desander, the pressure drop increases and separation performance improves.

The next article will discuss valve size and selection for desander use.

References

Rawlins, C.H., and Wang, I. I., “Design and Installation of a Sand Separation and Handling System for a Gulf of Mexico Oil Production Facility,” SPE Production and Facilities, paper 72999, Vol. 16, No. 3, 2001, pp. 134-140.